1. Field of the Invention
The present invention relates to the use of cobalt complexes as electron donor for the preparation of an active layer (heterojunction) in a photovoltaic conversion cell, and also to the multilayer photovoltaic conversion cell having an active layer comprising at least one such cobalt complex and at least one electron acceptor.
The field of the invention may be defined as that of organic semiconductors and in particular of heterojunctions.
2. Description of Related Art
The most common photovoltaic cells consist of semiconductors, mainly based on amorphous or monocrystalline silicon (Si). They are generally in the form of thin sheets having sides of about 10 cm, sandwiched between two metal contacts, for a thickness of the order of 1 millimeter. The best-performing silicon-based cells comprise an active layer of monocrystalline silicon, the conversion efficiency of which may achieve 40% in the laboratory.
Although they have a very high-performance, photovoltaic cells based on and in particular on monocrystalline silicon, have the major drawback of being expensive due to the high cost of this raw material. This is why some research turns to cells based on thin-film semiconductors.
Specifically, the thin-film technology makes it possible to reduce the amount of semiconductors used and furthermore enables the use of substrates of low cost and large surface area. In the thin-film cells, the silicon may be amorphous silicon or crystalline, in general polycrystalline, silicon. However, photovoltaic conversion cells based on thin films made of amorphous silicon are subject to stability problems when they are exposed to the sun. Furthermore, due to its disordered structure, the charge transport properties of the amorphous silicon are mediocre, hence a mediocre efficiency. Thus, a 10-50% drop in the efficiency of these cells occurs during the first hundreds of hours of exposure to light of the cells based on amorphous silicon.
Cells based on organic semiconductors, and in particular on organometallic compounds, the cost price of which is lower than that of silicon, have already been proposed. Their use in the photovoltaic field is based on the capacity of certain π-conjugated polymers and oligomers, or else of certain π-conjugated small molecules, to convert light energy into electrical energy. When a junction is formed that is composed of two semiconductors of different natures, at least one of which is an organic compound, a heterojunction is thus defined.
Heterojunctions comprising an organic semiconductor of p type and an organic or inorganic semiconductor of n type have for several years known many applications in the field of plastic electronics and especially in the particular field of photovoltaic conversion cells. Generally, in the latter, the π-conjugated polymer or oligomers, or the π-conjugated small molecule acts as a p-type donor and is brought into contact with an n-type acceptor such as for example fullerene, or a derivative thereof. Under light irradiation, an electron-hole pair is created (exciton) on the electron donor. This exciton is dissociated by capture of the electron by the acceptor. These charges are collected at the electrodes and generate an electric current.
Many heterojunctions have thus already been proposed in the literature.
Vanlaeka et al. (Solar Energy Materials and Solar Cells, 2006, 90(14), 2150-2158) describe, for example, a heterojunction for organic photovoltaic cells, consisting of a mixture of poly(3-hexylthiophene) (P3HT) as electron donor and of methyl[6,6]-phenyl-C61-butyrate (PCBM) as electron acceptor. C. J. Brabec et al. (Synthetic Metals, 1999, 102, 861-864). F. Silvestri et al, (J. Am. Chem. Soc., 2008, 130, 17640-17641) describe the preparation of heterojunctions of photovoltaic cells from a solution of certain squaraine derivatives used as electron donors, in combination with PCBM. However, these photovoltaic cells have conversion efficiencies which are around 50% lower than those of silicon-based cells.
It has already been envisaged to use certain metallic complexes in order to improve the performances of these photovoltaic cells. It is in this way that Z. Xu et al. for example (Journal of Applied Physics, 2008, 103, 043909-1-8) indicate that the conversion performances of a heterojunction composed of a mixture of poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) and of PCBM can be improved by the presence of phosphorescent molecules such as Ir(ppy)3 which is a complex of iridium and of tris(2-phenylpyridine). Finally, it has also already been proposed, in particular by W. K. Chan et al. (Newsroom, 2009, 10.1117/2.1200908.1757) to use, as active compound (electron donor), a heterojunction of photovoltaic devices, certain ruthenium complexes, as a mixture with a fullerene as electron acceptor. These devices do not however have a good conversion efficiency of light energy into electric current. Iridium and ruthenium are furthermore extremely rare and expensive elements.
Finally, it has already been proposed to use certain cobalt-based metallic complexes for applications in photovoltaics such as for example cobalt phthalocyanines, however these compounds have proved to be less effective than other metallic complexes such as nickel, copper or zinc complexes (Chamberlain, G. A., Solar Cells, 1983, 8, 47-83).